This invention relates to the art of optical micro-electromechanical systems (MEMS) devices, and more particularly, to all-optical switching using MEMS devices.
One solution for all-optical switching employs two MEMS devices each containing an array of tiltable micro mirrors, e.g., small mirrors, which can reflect light, which herein refers to any radiation in the wavelength of interest, whether or not in the visible spectrum. An optical path is established for light supplied from an input source, e.g., an optical fiber, to an output, e.g., an output fiber, by steering the light using a first micro mirror on the first optical MEMS device, the first micro mirror being associated with the input fiber, onto a second micro mirror on the second optical MEMS device which is associated with the output fiber. The second micro mirror then steers the light into the output fiber. Each fiber connected to the system is considered a port of the system, the input fibers being the input ports and the output fibers being the output ports.
Often, the light to be steered from the input fiber onto the first micro mirror of the first optical MEMS device first passes through a micro lens that is associated therewith and is part of an input micro lens array. The function of each micro lens is to collimate the beam of light supplied from its respective associated input fiber. Alternatively, in lieu of employing a separate micro lens array, a lens may be integrated with each fiber of fiber bundle in an arrangement that forms a collimator. A similar arrangement of a micro lens array or collimators are also found interposed between the output MEMS device and the output fiber bundle in the output section of the all-optical switch. In the output section, the function of each micro lens is to couple the light beam into its respective associated output fiber.
At present, the tilt angle of a micro mirror is set by applying to one or more electrodes appropriate voltages. Unfortunately, using current electrostatic angle control technology, the tilt resulting for a particular control voltage is a highly non-linear function. Consequently, requiring less tilt tends to allow the angle desired to be more accurately achieved. As a result, all-optical switches tend to include the functionality of a so-called xe2x80x9cfield lensxe2x80x9d between the MEMS devices. The field lens causes beams reflected from untilted mirrors on a MEMS device to converge. This acts to translates the angle at which the light is incident onto each micro mirror into a position to which the light will be directed upon reflection from the micro mirror, thereby allowing all the input micro mirrors to be homogenized. By homogenized it is meant that all micro mirrors having the same tilt will direct their light to the same position. Furthermore, the field lens refocuses each of the beams that pass through it, thus reducing loss. However, use of the field lens does not decrease the distance that is required between the input and output MEMS devices.
Because the all-optical switch is typically made up of sets of mirrors that cooperate to switch light from any input port to any output port, the entire system needs to be aligned to achieve the best possible optical connections, i.e., the least loss from the input to the output. To this end it must be determined what voltages need be applied to the electrodes controlling each mirror to achieve the best connection between it and each other mirror of the opposing set, and what voltage needs to be applied to the electrodes of each of the opposing mirrors as well. This process of determining the voltages is known as xe2x80x9ctrainingxe2x80x9d. When a field lens is used, the entire optical switch must be trained as a unit because there are variations from one system to the next due to variations in the respective field lenses and in the mounting position. This training process is time consuming and must be repeated should any component need to be replaced.
We have recognized that the field lens can be eliminated, while the same effect as if a field lens had been included in the system is maintained by, in accordance with the principles of the invention, causing the beams between a MEMS device and an input source or an output, e.g., a fiber bundle, to be closer to each other at the MEMS device than at the fiber bundle. This can be achieved in a variety ways. In one embodiment of the invention in which each fiber is associated with a respective micro lens of a micro lens array, by insuring that there is a different distance between the centers of adjacent micro lenses than there is for the centers of their corresponding adjacent fibers. In another embodiment of the invention in which the fibers are terminated by collimators, the direction of the collimators can be adjusted to point the beams in a converging manner. In yet another embodiment of the invention, an optical system that changes the direction of various ones of the beams may be interposed between a) the fiber bundle, and any associated micro lens array or collimators, and b) the corresponding MEMS device. Such an optical system could be any focusing lens arrangement, a multiple prism arrangement, and a multiple mirror arrangement where each mirror is tilted to point the beams in a converging manner. Furthermore, the optical system could be combined with an imaging system to image the micro lenses or collimators onto the MEMS device.